1. Field of the Invention
This invention relates to an improvement of method and apparatus for producing a binary information for an information transmission.
2. Description of the Prior Art
Concerning the transmission of informations through transmission lines, various coding schemes by which those transmission lines were usable with high efficiencies and also with as less deterioration of informations as possible have been devised and used so far.
As is well known in the art, a certain preferable scheme is chosen among such various coding schemes in dependence upon the types of information sources and the characteristics of the transmission lines, so as to avoid mismatching between the information sources and the transmission lines. From the above point of view, the coding scheme can be regarded as interfaces between the information sources and the transmission lines, and hence, in the information transmission apparatuses they are one of very important factors which determine their performance.
The general information transmission process is shown in a schematic block diagram of FIG. 1.
In FIG. 1, 1 is an information source, 2 is an encoder, 3 is a transmission line, 4 is a decoder, and 5 is an information receiving end. When an information generated in the information source 1 is transmitted through the transmission line 3 to the information receiving end, first of all, the information is applied to the encoder 2 and transformed thereby, then it is applied to the transmission line 3. The information passed through the transmission line 3 is inversely transformed to the original pattern by the decoder 4 and thus it reaches the information receiving end 5. As has been pointed out, it is often the case that the information source 1 and the transmission line 3 mismatches. For example, it might happen such that the occupied frequency band width of an information sent out from the information source 1 differs from the pass-band of the transmission line 3. Then for accomplishing an accurate information transmission within the pass-band of the transmission line 3, the information from the information source 1 is to be transformed by the encorder 2.
The prior arts of the coding schemes are shown in FIG. 2 with using corresponding waveform diagrams. In FIG. 2, 6 is a data sequence, 7, 8, 9, 10 and 11 show various coded waveforms of said data sequence 6 obtained through different coding schemes, NRZ, NRZI, FM, PE, and MFM, respectively. Here, u indicates the time length of one bit-cell, that is, the bit-cell length.
As is well known, the NRZ(7) is a coding scheme wherein only when the data (6) is "1" (corresponding to "high"), the output is kept to high level during its bit cell period, while NRZI(8) is a coding scheme wherein only when the data 6 is "1" the output level is inverted. Meanwhile, in the case of FM(9), when the data (6) is "0" the output level is inverted only at the boundary of its bit-cell and when the data (6) is "1" the output level is inverted at the center as well as at the boundary bit-cell. In the case of PE (10), when the data (6) is "1" the output level rises up at the center of its bit-cell, and when the data (6) is "0" the output level falls down at the center of its bit-cell. The inversion of the output at the bit cell boundary in this scheme taken place in dependence upon the relative sequence of the data (6). In the case of MFM (11), when the data (6) is "1" the output level is inverted at the center of its bit-cell, and when the data (6) is kept "0" over successive bit-cells, the output level is inverted at the beginning of the bit-cell. These coding schemes mentioned above are only several examples among the schemes hitherto used. Here, in either case of those coding schemes shown in FIG. 2, for every bit of the input data, one unit of predetermined output waveform is given.
In contrast to the abovementioned coding schemes, many different coding schemes have been devised and used in practice, wherein the input data sequence is divided into blocks at every prescribed number of bits and the encoding is processed by converting those data blocks into different data blocks in accordance with data pattern contained in each data block. As those coding schemes, the 4-5 conversion, the 8-9 conversion, and 8-10 conversion which are seen in the IBA coding may be mentioned. These are called by general terms such as "block coding" or "GCR". In either case, the input data sequence is divided into blocks, and resulted data patterns contained in the blocks are converted new data patterns corresponding to its original data patterns in a prescribed manner.
Then, as an example of the block coding, case of the 8-10 conversion is shown in FIG. 3.
In FIG. 3, (13) is a clock signal corresponding to an input data sequence applied to the encoder. Hereinafter the clock signal is represented by a row of upward-directing arrows as in FIG. 3. Every arrow represents the time at which the clock signal rises up or falls down to define the clock time. (14) is an input data sequence, (15) is a clock signal corresponding to an output data sequence produced from the encoder, (16) is an output data sequence, v is a bit-cell period of the input data, v' is a bit-cell period of the converted output data, w is an input data block, w' is a converted output data block, a.sub.n0 to a.sub.n7 are data extending over 8 bit-cells, in which suffix n represents an n-th data block in the input data sequence, and b.sub.n0 to b.sub.n9 are data extending over 10 bit-cells and constitute an n-th data block in the converted output data sequence. As is clear also from FIG. 3, the number of possible data patterns in the input data block is 256 (2.sup.8), and on the other hand, there exists exactly 1024 (2.sup.10) different data patterns in the data output block. That is, the number of data patterns in the output data block exists four (2.sup.2) times as much as the number of possible data patterns in the input data block, then there is a freedom of selecting 256 different data patterns out of 1024 usable patterns as the output data block, depending upon a desired way of setting the character of the converted data patterns.
Also, as has been pointed out, an adequate coding scheme must be selected in dependence on the characteristics of transmission line utilized. In usual cases, the transmission lines have a band-limited characteristic. Hereupon, the word of transmission line in this invention is used not only in a sense of ordinary means but also in a sense including apparatuses, to and from which the coded signal is fed and taken out. For example, a magnetic recording and reproducing system might be mentioned as the transmission line used in the above broad sense.
Therefore, in this specification, the explanation is given by taking an example in which a magnetic recording and reproducing system is considered as the transmission line.
The magnetic recording and reproducing system has normally the band-limited characteristic due to those factors such as the characteristics of transducers for transforming magnetic signals into electric signals, and vice versa or various existing losses. Therefore, for the magnetic recording and reproducing, an ideal coding scheme is such that through which signal can be converted into a signal containing neither dc nor extremely low frequency components and having a frequency spectrum concentrating inside the signal pass-band. In more concrete description, an ideal coding scheme is such that through which signal's minimum interval of the magnetizing inversion is long and its maximum interval of the magnetizing inversion is short and also its dc component is eliminated after the conversion. If other type of recording or transmission than the magnetic one, the abovementioned "magnetizing inversion" should read "electric inversion".
In the above explanation, expressions such as "containing no dc component" or "dc component is eliminated" has been used. Hereupon, it will be necessary to clarify the meaning of the above expressions. It is as follows: When a certain signal is expressed by, g(t), a function of time, the meaning of the expression "containing no dc component" is g(t) is a function of upper bounded type in integrated form". That is, it means a signal expressed by g(t) satisfying the following equation: EQU .vertline..intg.g(t)dt.vertline..ltoreq.Y, (101)
where Y is an arbitrary constant. Using still another expression, those having characteristics expressible by Eq. (101) can be called as "balanced code scheme".
Then, in the following explanation, instead of the expression "containing no dc component", the term "balanced code" will be used and furthermore, since only binary signals are treated in this invention, particularly a term "binary balanced code" will be used.
As is clear from the above explanation, upon evaluating various coding schemes, the maximum and minimum values of the interval of the magnetizing inversion and whether it is a balanced code or not are most important factors. Then, about each coding scheme shown in FIG. 2 and FIG. 3, evaluated results on the above three points are listed in TABLE 1.
TABLE 1 ______________________________________ maximum interval minimum interval coding of magnetizing of magnetizing balanced scheme inversion inversion code or not ______________________________________ NRZ infinity 1 no NRZI infinity 1 no FM 1 1/2 yes PE 1 1/2 yes MFM 2 1 no 3 PM 6 3/2 no IBA 8 4/5 yes 8-10 conversion ______________________________________
In TABLE 1, as an example of the block coding, the "IBA 8-10 conversion" is shown. Also in this table, the maximum and minimum interval of the magnetizing inversion are tabulated with those values normalized by the bit-cell period of the input data.
As is clear from TABLE 1, FIG. 2 and FIG. 3, in NRZ or NRZI, although the minimum interval of the inversion (hereinafter denoted as Tmin) takes a comparatively large value, that is, 1, the maximum interval of the inversion (hereinafter denoted as Tmax) becomes infinity, and accordingly neither of them is a binary balanced code. In FM and PE, although they are binary balanced codes, Tmin is as very small as 1/2. In MFM, Tmin is 1 and Tmax is 2 and hence they are close in their values to each other, however, it is not a binary balanced code. Also in 3 PM, Tmin and Tmax take separated values as 3/2 and 6, respectively, and even more it is not a binary balanced code. Meanwhile, in the case of IBA 8-10 conversion, since it is devised in a manner that dc component is not included upon the conversion of data block, it is naturally a binary balanced code, however Tmin is 4/5, being less than 1, while Tmax is 8, which is a considerably large value.
In TABLE 1, only several examples of coding schemes were shown. For the evaluation of these coding schemes for the magnetic recording and reproducing system, the same requirements are proposed as mentioned before, that is, Tmin should be as large as possible from the viewpoint of recording density and also Tmax should be as small as possible from the viewpoint of self-locking. Also, since, in ordinary magnetic recording with rotating heads, and reproducing systems, the dc component cannot be transmitted, the binary balanced code is quite effective for such the magnetic recording and reproducing system. However, in the conventional binary balanced codes proposed so far, such as in FM, PE, IBA 8-10 conversion, or ZM, Tmin in either of them is 1 or less than 1.
As described already, from the requirement of raising the reproducibility of clock signal at the receiving side, and also from the requirement of reducing the low-frequency component after the conversion, the maximum interval of the inversion Tmax should be as small as possible. Meanwhile, from the requirement of reducing the high-frequency component after conversion, the minimum interval of the inversion Tmin should be as large as possible. Furthermore, in the usual transmission lines, it is often the case that the dc component cannot be sent through them, particularly in the magnetic recording and reproducing systems wherein the transmission lines are regarded as magnetic tape-recording head systems, any coding scheme not including dc component is strongly desired.